1. Field of the Invention
This invention relates to improvements in a dry-processing apparatus for processing the surface of a workpiece using a vapor phase, and more particularly to prevention of degassing from and reflection of active particles at the inner wall of the reaction chamber of an apparatus of the kind above-described.
2. Description of the Prior Art
FIG. 1 shows the fundamental structure of a prior art dry-processing apparatus. Referring to FIG. 1, the prior art dry-processing apparatus is composed of a gas decomposing chamber 1 provided with gas decomposing means such as an external coil and internal parallel planar electrodes, gas introducing means 2 such as an etching gas supply pipe provided with a leak valve, a reaction chamber 3, a workpiece support 5 disposed in the reaction chamber 3 for supporting a workpiece 4 thereon, and an evacuation system 6. The workpiece 4 is not necessarily placed in the reaction chamber 3 and may be placed in the gas decomposing means. The functions of the gas decomposing means include:
(1) to decompose molecules of the introduced gas;
(2) to excite molecules of the introduced gas or their decomposition products; and
(3) to effect an ion and/or molecule reaction such as polymerization of molecules of the introduced gas or their decomposition products.
The gas decomposing means is classified into two types which are: (1) means for producing plasma in a portion of the reaction chamber 3 or in the gas decomposing chamber 1 connected to the reaction chamber 3; and (2) means for introducing light into a portion of the reaction chamber 3 or into the gas decomposing chamber 1 for decomposing the gas with the light. The former means, that is, the means for producing the plasma includes DC discharge (1), RF discharge (2), microwave discharge in the absence of a magnetic field (3), microwave discharge in the presence of a magnetic field (4), etc. And, the light introduced for gas decomposition includes infrared radiation (CO.sub.2 laser having a wavelength of, for example, about 9 to 10 .mu.m), visible radiation (Ar laser having a wavelength of, for example, about 0.5 .mu.m), etc (5). From the aspect of principle, ultraviolet radiation having a wavelength not longer than 0.2 .mu.m is also usable.
(1) Yoshihiro Hamakawa: Solid-state Phisics (Kotai Butsuri, Japan) 14-10 (1979), pp. 43-53
(2) Tsutomu Tsukada et al.: Vacuum (Shinku, Japan) 23-9 (1980), pp. 1-10
(3) Takuo Sugano (Editor): "Application of Plasma Process to VLSI Technology" pp. 120-138, published by Sangyo Tosho Ltd.
(4) The same, pp. 138-153
(5) T. J. Chuang: IBM J. RES. DEVELOP. 26-2 (1982), pp. 145-150
In the dry-processing apparatus having such a construction, active particles (ions and neutral radicals) produced in the gas decomposing chamber 1 and arriving at the surface of the workpiece 4 make physical and chemical reactions with the workpiece surface 4 thereby effecting surface processing such as etching or deposition. In this case, it is needless to mention that the gas to be decomposed is introduced into the gas decomposing chamber 1 by the gas introducing means 2.
A problem encountered in the case of etching of the workpiece 4 by the use of such a dry-processing apparatus will be described with reference to FIGS. 2A and 2B.
In FIG. 2A, the workpiece 4, which is a semiconductor element, is subjected to isotropic etching through a mask 4' and has a sectional shape as shown after etching. On the other hand, in FIG. 2B, the workpiece 4 is subjected to directional (vertical) etching through the mask 4' and has a sectional shape as shown after etching. In order that the workpiece 4 can be most finely processed according to the pattern provided by the mask 4', the vertical etching meeting exactly the masking pattern 4' is fundamentally more preferable than the isotropic etching. (A technique which can freely control any desired intermediate state between the vertical etching and the isotropic etching will be more useful.)
As described already, ions and neutral radicals are present as active particles contributing to the process of vapor phase etching. Among these active particles, the ions can attain the vertical etching since the ions are vertically incident upon the surface of the workpiece 4 by being accelerated by the electric field. However, the neutral radicals provide the isotropic etching, since the neutral radicals are free from the influence of the electric field and are isotropically incident upon the surface of the workpiece 4 by being reflected by the inner wall of the reaction chamber 3. On the other hand, vapor phase etching with the accelerated ions tends to impart damage to the surface layer of the workpiece 4, but such damage to the surface layer of the workpiece 4 does not occur in the case of vapor phase etching with the neutral radicals.
Therefore, in a dry-etching apparatus, especially, that for processing a semiconductor element vertical etching with neutral radicals is especially desirable.
A problem as pointed out below is encountered when the dry-processing apparatus shown in FIG. 1 is used as a deposition apparatus (a film forming apparatus). The active particles produced in the gas decomposing chamber 1 impinge against the inner wall of the reaction chamber 3 to release atoms and molecules (mainly, those of H.sub.2 O, O.sub.2, N.sub.2 and CO.sub.2) which are adsorbed to the inner wall of the reaction chamber 3 or which react with the material of the inner wall of the reaction chamber 3 to release a portion of the wall material into the vapor phase, and such released atoms and molecules or such a released wall material become mixed with the deposited film to obstruct formation or deposition of a film of high purity. For example, when a gas mixture of (SiH.sub.4 +N.sub.2) introduced into the gas decomposing chamber 1 by the gas introducing means 2 is decomposed to deposit a film of Si.sub.3 N.sub.4 on the surface of the workpiece 4, atoms and molecules of H.sub.2 O, O.sub.2 and CO.sub.2 adsorbed to the inner wall of the reaction chamber 3 are released to mix in the vapor phase, with the result that a mixed film of Si.sub.3 N.sub.4 and SiO.sub.2 will be deposited on the surface of the workpiece 4. Therefore, a deposition apparatus is desired in which such atoms and molecules adsorbed to the inner wall of the reaction chamber 3 or the material of the inner wall of the reaction chamber 3 would not mix into the vapor phase.